Image storage device



WRITE GUN 0d. 27, 1970 s. R. SHORTES 3,535,949

IMAGE STORAGE DEVICE Filed Sept. 9, 1968 2 Sheets-Sheet 1 READ OUTPUT FROM i READ GUN INVENTOR SAMUEL R. SHORTES Fl. 4 e V READ OUTPUT 6*? m ATTORNEY Filed Sept. 9, 1968 FROM WRITE GUN LOW ENERGY HIGH ENERGY s R- SHORTES IMAGE STORAGE DEVICE READ OUTPUT LIGHT 2 Sheets-Sheet 2 READ OUTPUT INVENTOR SAMUEL R. SHORTES ATTORNEY US. Cl. 315-11 11 Claims ABSTRACT OF THE DISCLOSURE A solid state device is provided for storing images which includes a body of semiconductor material having opposed surfaces. A plurality of discrete areas of the opposite semiconductor type are disposed on one of the opposed surfaces and a layer of phosphorescent material is disposed on the other of the opposed surfaces. Images are transmitted by a write electron source to the layer of phosphoresecent material which luminesces to provide representations of the images to the body of semiconductor material for a preselected time interval. The images cause variance of the electrical charge of the discrete areas. A read electron source then scans the discrete areas to provide electrical indications of the images.

This invention relates to the storage of images, and more particularly to an image storage device suitable for use with scan converter systems and the like.

Scan converters are commonly used in order to change scan formats. For example, it is commonly desirable to change a circular scan to a horizontal television-type scan. One type of scan converter heretofore developed utilizes a target comprising a thin copper mesh with calcium fluoride or zinc sulfide applied to portions thereof. The target is disposed between two opposed electron guns, with one electron gun used to write an image upon the target by varying the electrical charge density of areas of the target according to a scan pattern. The second electron gun then scans the charge density pattern of the target with a different scan pattern. A read collector grid collects electrons from the second electron gun which are reflected by the target in order to provide an indication of the stored image. Problems have arisen in the use of such scan converters due to the introduction of spurious cross-talk signals into the output because of interaction between the opposed electron beams which pass through the mesh target.

In an effort to eliminate problems inherent with wire mesh targets, and in particular cross-talk, scan converters have been developed which utilize photon coupling through a target. Examples of such devices include the use of a cathode ray tube in combination with a fiber optics faceplate which transmits the tube image to a vidicon tube. However, the cost of such fiber optics faceplates are prohibitiyely high for use in many applications. Other systems have utilized a cathode ray tube coupled to a television camera, but problems have arisen in obtaining satisfactory resolution for many applications with such systems.

Another type of image storage device is described in US. Pat No. 3,011,089, entitled Solid State Light Sensitive Storage Divice, issued to F. W. Reynolds on Nov. 28, 1961. This solid state device utilizes an array of discrete p-n junctions which may be selectively capacitively charged and discharged to store information. Although this device provides may advantages over the previously described devices, the storage time of such solid state devices has been limited generally to the lifetime of hole carriers, thereby limiting the practical applications of the devices.

States to In accordance with the present invention, a body of semiconductor material having opposed surfaces is provided with a plurality of discrete areas on one of the opposed surfaces, with each of the areas having a different electrical charge distribution than the semiconductor material. Upon the reception of images on the other of the opposed surfaces of the smiconductor body, the electrical charge of the discrete areas is varied to provide a representation of the images thereon. In order to provide storage of the received images for a substantial time, structure is provided on the body of simiconductor material to provide indications of the images to the discrete areas after the reception of the images.

In a specific aspect of the invention, the present storage device is constructed from n-type semiconductor material with a plurality of discrete areas of p-type semiconductor material being disposed on one surface thereof. A write source transmits images to a layer of phosphorescent material, while a read electron source scans the discrete areas of p-type material to provide scan conversion. The intensity of the write source may be varied in order to vary the presistence of the image storage of the device.

For a more complete understanding of the invention and for further objects and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a scan converter according to the present invention;

FIG. 2 is a diagrammatic cross-section of a portion of the present image storage device;

FIG. 3 is a back view of the device shown in FIG. 2;

FIGS. 4 and 5 are diagrammatic illustrations of the operation theory of the present device;

FIG. 6 is a graph illustrating characteristics of phosphorescent materials; and

FIG. 7 is a diagrammatic illustration of another aspect of operation of the present device.

Referring to FIG. 1, a scan converter designated generally by the numeral 10 comprises a write electron gun 12 which includes a focus coil 14 and a deflection yoke 16. Collimators 17 are disposed within a highly vacuumized glass tube 18 in order to properly focus the beam of electrons from gun 12. The present target and storage device 19 is disposed midway between the write gun 12 and a read electron gun 20. A focus coil 21 allows focusing of the electron beam from the read gun 20, while a deflection yoke 22 provides a desired scanning motion to the electron beam.

A pair of collimators 23 assist in properly focusing the read electron beam upon the target 19. It will be seen that the general configuration of the present scan converter 10 is somewhat similar to previous scan converters, with the exception of the novel target device 19 which eliminates the necessity for write and read collector elec trodes and the like. Examples of suitable read and write electron guns are the guns utilized in scan converters manufactured and sold as Model H-1161 and H-1203 by the Hughes Aircraft Company of Los Angeles, Calif.

Referring to FIG. 2, a relatively thin body 24 of semiconductor material is preferably made from a Wafer of n-type silicon. A plurality of discrete areas 26 are de fined in the side of the semiconductor body 24 which receive the beam from the read electron gun, Discrete areas 26 preferably comprise relatively heavily doped p-type diffusion areas somewhat similar to diffused regions in a MOS device. As shown in FIG. 3, the discrete areas 26 are symmetrically spaced across the face of the target 19. The size and spacing of the discrete areas 26 will be dependent upon the desired resolution of the target 19.

A layer 28 of phosphorescent material is disposed on the surface of the semiconductor body 24 which faces the beam from the write gun 12. As will be later described in detail, the layer of phosphorescent material 28 increases the storage time of the memory device. An output sensor 30 is connected to the semiconductor body 24 in order to sense electrical indications of the image received by the device 19. When the device 19 is used in a scan converter, the scanning operation of the read gun 20 is synchronized with the output sensor 30 to provide scan conversion.

The device 19 may be fabricated according to a number of processes well-known in the art. For example, oxide may be deposited over a polished silicon wafer. Holes are then etched through the oxide layer and the p-type discrete areas are diffused into the silicon wafer. Target 19 will normally be extremely thin, with the preferred embodiment having a thickness in the micron range.

FIGS. 4 and 5 diagrammatically illustrate the operation theory of the target 19. Initially, as shown in FIG. 4, a beam of electrons 31 from the read gun 20 is scanned across the back face of the device 19 according to a preselected scan pattern. The negative charges of the electrons from the read gun beam 31 reverse biases each of the discrete areas of p-type material to form depletion layers designated generally by the numerals 32a and 32b. Electron beam 31 is scanned over the complete back face of the device 19 with a constant energy level such that each of the discrete areas 26 are charged to a reference negative charge with respect to the n-type semiconductor body 24.

The plurality of p-n junctions provided in the device 19 may be seen to provide a plurality of semiconductor diodes whose capacitance may be varied by the impingement of a beam of electrons thereon. After each of the discrete areas 26 has been reversed biased by the read gun 20, the device 19 is in condition to store an image transmitted from the write gun 12.

As shown in FIG. 5, an electron beam, or light beam 34, is scanned across the front face of the device 19 according to a preselected scan pattern. The energy level of beam 34 is modulated during the scanning of the target 19 in order that the desired image is beamed upon the target 19 after one complete scan of the target. The electrons from the beam 34 are absorbed by the phosphor layer 28, which luminesces to provide representations of the image to the semiconductor body 24 for a substantial time after the beam 34 has moved to a different location.

FIG. 6 illustrates the well-known secondary emission characteristics of a phosphor wherein after light, or an electron beam, is impinged upon a layer of phosphor during the time interval 0 to t a substantial amount of light or electrons is emitted from the phosphor for a substantial time period thereafter. This characteristic of the phosphor layer 28 enables the reading of the device for a substantial time after the reception of the image, thereby providing operational flexibility to the device.

In response to the secondary emission by the phosphorescent layer 28, the n-type semiconductor body 24 generates hole-electron pairs in a manner shown diagrammatically in FIG. 5. Due to the reference reverse bias previously applied to the discrete areas 26, the holes thus generated diffuse throughout the n-type semiconductor body and are collected by the discrete areas 26.

The electrical charges of the discrete areas 26 are then varied from a reverse biased condition to a more positive charge condition.

The magnitudes of the positive charges imparted to the discrete areas are dependent upon the energy level of the beam 34, and thus any desired amount of tone graduation may be achieved. For instance, as shown in FIG. 5, discrete area 26a is shown as being restored to a substantially positive charge condition by the impingement of the beam 34. However, the discrete area 26b has not been subjected to the beam 34, and thus remains in the original reversed biased reference condition. Other discrete areas 26 will be subjected to different energy levels by the beam 34 and will thus have electrical charges different from the discrete areas 26a and 2612.

After the write gun beam 34 has been scanned across the front face of the target 19, the read gun 20 again scans the back side of the target 19, as shown in FIG. 4, in order to provide sensing of the image transmitted from the write gun. When the beam 31 from the read gun impinges upon a discrete area 26 whose electrical charge has been varied by the collection of hole carriers, the read gun beam 31 again reverse biases the discrete area 26 back to the reference level. This recharging of the n-p junction diode by the read gun beam 31 generates an alternating current signal in the n-type semiconductor body 24 which is sensed by the output sensor 30.

The alternating current signal sensed by the output sensor 30 is representative of the image transmitted by the Write gun 12. For instance, when the read gun beam 31 impinges upon a discrete area 26 which has been substantially uncharged by the impingement of a high energy level beam from the write gun, a substantial amount of chargining by the read gun beam will be required to again reverse bias the discrete area. Thus, a relatively large alternating current signal will be generated. However, when the read gun beam impinges upon a discrete area 26, which the write gun beam has not impinged upon, little or no charging of the discrete area occurs, and thus a relatively low or no alternating current output is generated. For additional disclosure of the capacitive storage provided by the p-n junctions, reference is made to the previously described US. Pat. No. 3,011,089.

The scanning of the read gun beam 31 is synchronized with the sensing output 30, in order that the output sensor 30 Will provide a representation of the transmitted image according to the scanning pattern of the read gun. In this manner, scan conversion between the write gun 12 and the read gun 20' is accomplished.

It will be understood that various types of phosphors may be utilized for the present invention, the only requirement being that the phosphors provide a suitable secondary emission to provide the desired storage interval. Different types of phosphors will thus be chosen for various applications. For instance, phosphors may often be chosen with decay rates fast enough to allow new writing after a relatively short time. For other applications, relatively long decay rates will be required to allow substantial delayed reading. Examples of suitable phosphors for use with the invention are any of the P-22 phosphors commonly used in television systems. For other applications, such phorphors as lead activated barium silicate will be desirable.

It will be understood that the thickness of the n-type semiconductor body 24 will be harmonized with the emission wavelength of the phosphor layer 28 in order to provide the desired resolution. The secondary emission wavelengths of the phosphor layer will be chosen in accordance with the desired use. A distinct advantage of the invention is that phosphor responsive to infrared light may be utilized in order to provide an infrared sensitive system.

It will be understood that the present invention provides substantial advantages. Cross-talk between the read and write guns is substantially eliminated. Due to the storage provided by the phosphor layer, relatively long delays between write and read scans may be utilized. If desired, integration of successive images may be provided by adding or writing new images upon the image stored upon the phosphor layer. Relatively high voltages may be utilized for the write gun, and thereby gains in signal current from the write to read guns may be realized. The target 19 should have a thickness such that the electrons from the write gun will not penetrate directly to the discrete areas 26, but the target should be thin enough that the generated hole carriers may easily diffuse to the p-type regions.

For normal use of the present target 19, the write gun 12 is biased to deliver relatively low energy electrons in a manner similar to a conventional vidicon tube. FIG. 7 illustrates the generation of hole-carriers by such a low energy beam, the hole-carriers being collected by the ptype semiconductor region 2601. However, in some instances it is advantageous to provide a relatively high energy write beam sufficient to penetrate the thickness of the phosphor layer 28. In such penetration of the phosphor layer 28, relatively no absorption by the layer 28 occurs, and thus substantially no secondary emission occurs. Hence, a relatively large number of hole-carriers are directly generated in the n-type semiconductor body 24, but relatively no persistence of the high energy beam occurs after the lifetime of the hole-carriers.

It is thus possible to control the persistance of the transmitted image by utilizing this concept of the invention. Such control is of particular advantage for use in airport type radar systems wherein it is desirable to have persistence of airplane blips, but it is not desirable to have persistence of tags or labels which identify a particular airplane blip on the radar screen. With the use of the present target, relatively low energy beams are utilized to provide indications of the position of airplanes, while relatively high energy beams are utilized to provide tags identifying the airplanes, thereby eliminating any persistence of the tag images which would tend to clutter the screen.

While specific embodiments of the present invention have been described in detail, it will be understood that various changes and modifications may be suggested to one skilled in the art, and it is desired to encompass such changes and modifications as fall within the scope of the invention.

What is claimed is:

1. An information storage device comprising:

(a) a body of semiconductor material having first and second opposed surfaces,

(b) a plurality of discrete areas on said first surface each operable to change electrical charge distribution upon the reception of images by said second surface, and

() storage means on said second surface for receiving images and operable to emit continuing representations of said images to said discrete areas for a preselected time interval after the reception of said images.

2. The device of claim 1 wherein said discrete areas comprise:

a plurality of areas on said first surface wherein a different type of semiconductor material has been disposed.

3. The device of claim 1 wherein said means comprises:

a layer of phosphorescent material.

4. A device for storing images comprising:

(a) a body of a first type of semiconductor material having opposed surfaces,

(b) a plurality of discrete areas of a second type of semiconductor material disposed on one of said surfaces, and

(c) a layer of phosphorescent material disposed on the other of said surfaces.

5. The device of claim 4 wherein said first type of semiconductor material comprises n-type material and said second type of semiconductor material comprises ptype material.

6. In a scan converter wherein images transmitted by a write source are read by structure including a read electron source, the combination comprising:

(a) a thin body of semiconductor material having first and second opposed surfaces forming one plate of a plurality of capacitive elements,

(b) a layer of phosphorescent material disposed on said first surface for receiving images from said write source, and

(c) a plurality of discrete areas of material disposed on said second surface forming the other plates of said capacitive elements which provide indication of said images when scanned by the read electron source.

7. The combination of claim 6 wherein said layer of phosphorescent material has a preselected emission wavelength, said body of semiconductor material having a thickness dependent upon said emission wavelength to provide a predetermined image resolution.

8. The combination of claim 6 wherein the write source selectively transmits images having a first energy level suflicient for absorption by said phosphorescent material and images having a second energy level sufliciently high for penetration of said phosphorescent material without substantial absorption thereby.

9. The method of storing images consisting of the steps of:

(a) receiving transmitted images on one surface of a target,

(b) generating and storing in a phosphor material representations of said images for a predetermined time after the termination of said transmitted images,

(c) varying electrical characteristics of discrete portions of an opposed surface of the target in response to said representations, and

(d) sensing the varied electrical characteristics to provide an indication of said transmitted images.

10. The method of claim 9 wherein said step of varying electrical characteristics consists of:

scanning said discrete portions with an electron source to vary the electrical charge of said discrete portions.

11. The method of scan conversion consisting of the steps of:

(a) scanning a target surface with an image according to a first scan pattern,

(b) generating and storing representations of said image for a preselected interval in a phosphorescence material after said scanning terminates,

(c) varying the electrical charge distribution of an opposed surface of said target in response to said representations of said image,

(d) scanning said opposed surface with a source of electrons according to a second scan pattern to vary the electrical charge distribution of said opposed surface, and

(e) sensing the variance of the electrical charge distribution by said second scan pattern to provide a scan converted representation of said image.

References Cited UNITED STATES PATENTS 3,403,284 9/1968 Buck et al. 31511 3,440,476 4/ 1969 Crowell et a1 3 l510 3,440,477 4/1969 Crowell et a1. 31511 RODNEY D. BENNETT, Primary Examiner I. G. BAXTER, Assistant Examiner U.S. Cl. X.R. 

